1. Field of the Invention
The present invention relates to a power semiconductor device and, more particularly, to a compression bonded semiconductor device in which a breakdown voltage of the semiconductor device is increased.
2. Description of the Related Art
In a conventional power semiconductor device, in order to satisfy conditions such as heat dissipation, current capacity, breakdown resistance, and a diameter of a semiconductor pellet, a compression bonded package such as a stud or flat package is used. However, since a special junction edge region such as a beveled structure is used as a junction structure of the semiconductor pellet to obtain a higher breakdown voltage, it is difficult to compress the pellet to contact all surfaces of portions which generate heat during operation. According to the above compression bonded package, in the surface of the semiconductor pellet which generates heat, heat dissipation of a portion with which a compression bonded electrode is in tight contact is excellent, but heat dissipation of a portion with which the compression bonded electrode is not in contact is very poor.
In a conventional compression bonded semiconductor device using such a compression bonded package, as countermeasures to heat dissipation, the area of a portion where an external electrode is in contact with a semiconductor pellet is maximized, or a heat buffer plate made of molybdenum (Mo), tungsten (W), or the like, which has a thermal expansion coefficient close to that of the semiconductor pellet (silicon pellet) is fixed on one surface of the semiconductor pellet through an Al-Si alloy film.
Even when the above structures are used, heat dissipation of a portion with which an external electrode is not in contact is poor, and a semiconductor element may be destroyed.
FIGS. 1A and 1B schematically show a conventional alloy compression bonded diode and a conventional alloy free compression bonded diode, respectively. In FIGS. 1A and 1B, reference numeral 10 denotes a semiconductor pellet (silicon pellet) on which a p-n junction diode is formed, and main electrodes are formed on one (first) major surface and the other (second) major surface, respectively. An encapsulating member (e.g., silicone rubber) 11 is provided to cover the edge surface portion of the semiconductor pellet 10.
In the alloy structure shown in FIG. 1A, one surface of the semiconductor pellet 10 is fixed on a heat buffer plate (e.g., Mo) 12 by alloy bonding, and an electrode member 13 is arranged on the other surface of the semiconductor pellet 10. The semiconductor pellet 10 is compressed from both the sides by electrode posts (e.g., Cu) 14.
In the alloy-free structure shown in FIG. 1B, electrode posts 14 compress the electrode members 13, which in turn compress the the two electrodes 13 formed on the first and second major surfaces of the semiconductor pellet 10, respectively. The pellet 10 is compressed between the electrode members 13, but not fastened thereto. Both electrode member 13 are compressed between the pellet 10 and the electrode post 14, but are not fixed to the pellet 10 or the post 14.
Although portions of edge regions which are not covered with the electrodes 13 of the semiconductor pellets 10 shown in FIGS. 1A and 1B and which are indicated by arrows a (portions of the surfaces of the semiconductor pellets 10 with which external electrodes are not in contact) generate heat during an operation time, the portions have poor heat dissipation. For this reason, when the semiconductor pellet 10 is a diode, the edge region which is not covered with the electrode 13 and which is indicated by the arrow a is easily destroyed by a back power at a reverse recovery time.
As has been described above, according to a conventional compression bonded semiconductor device, a portion with which an external electrode is not in contact within the surface of a semiconductor pellet has poor heat dissipation, and the breakdown voltage of a semiconductor element is decreased.